Aluminum foil is used as a current collector in secondary batteries, such as lithium-ion batteries, in electric double-layer capacitors, and the like. For example, in the case of a lithium-ion battery, the positive electrode is configured by fixing a positive electrode active material to an aluminum foil surface.
The above-mentioned positive electrode is manufactured, for example, as described below. That is, coating layers are formed by coating both surfaces of the aluminum foil, whose thickness is approximately 15 μm, with a paste that is prepared by dispersing a positive electrode active material powder such as lithium cobalt oxide, a binding agent such as polyvinylidene fluoride, an auxiliary conductive agent such as carbon black, and the like, in an organic solvent such as N-methylpyrrolidone, and then mixing such. Subsequently, by drying the coating layers, the organic solvent in the coating layers evaporates and is removed. After the above-mentioned drying, a press bonding process for increasing the intralayer density is performed as needed. In so doing, a positive electrode is manufactured wherein the surfaces of the aluminum foil serving as the current collector have a positive electrode active material-containing layer.
As discussed above, in the case wherein the electrode is configured by fixing the electrode active material to the current collector surfaces, it is critical that the current collector and the electrode active material-containing layers be in sufficiently close contact with one another. This is because, in the electrode manufacturing process, if the electrode active material-containing layers peel off of the current collector, then there is a risk that the yield will decrease; in addition, if that peeling occurs after incorporation in a secondary battery, an electric double-layer capacitor, or the like, then the characteristics, such as the lifespan, of the device will adversely degrade.
As a method that improves the adherence between the current collector and the electrode active material-containing layers, a method is known that roughens the surface of aluminum foil. For example, in Patent Document 1 an aluminum foil for a current collector is disclosed wherein a mean roughness Ra, which serves as the roughness of at least one surface and is based on JIS B 0601:1994, is greater than or equal to 0.3 μm and less than or equal to 1.5 μm, and a maximum height Ry is greater than or equal to 0.5 μm and less than or equal to 5.0 μm.
In addition, there are techniques that improve the adherence between the current collector and the electrode active material-containing layer by methods other than roughening. For example, in Patent Document 2 an aspect is disclosed wherein the coating ability of the paste is improved and the adherence of the electrode active material-containing layer is improved by sufficiently removing the rolling oil that adheres to the aluminum foil surface after foil rolling.